As an example of a communication protocol related to an optical communication backbone network which transmits mass information over a long distance, an Optical Transport Network (OTN) is known which is defined in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) G.709. In the OTN, a client signal conforming to specific communication standards such as, for example, the Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH), and Ethernet® is arranged in a frame having a fixed length and is transmitted on a network by using a wavelength division multiplexing technology. In a network conforming to the OTN, a transmission path is not fixed, differently from a network conforming to the SONET/SDH, and may be changed during operation of the network by, for example, extension of a transmission device. The transmission path is selected and switched to provide an optimal transmission quality of the transmission path in the network.
Path delay time is one of factors indicating the quality of the transmission path. The path delay time is measured, for example, in a steady manner. In addition, there is a recent case where a communication carrier and a contractor sign a “Service Level Agreement” according to which the communication carrier explicitly guarantees the contractor for the quality of service. In this case, it may be important for the communication carrier to measure the path delay time for management of network quality. For example, if different transmission devices facing with each other on the transmission path are managed by different communication carriers, each of the different communication carriers measures the path delay time through the transmission device managed by the corresponding communication carrier and performs a maintenance and management of the network.
In the OTN, a Delay Measurement (DM) function is defined in order to measure the path delay time in an Optical Data Unit (ODU) Path Monitoring (PM) layer and the path delay time in a Tandem Connection Monitoring (TCM) layer. FIG. 1 is a view illustrating the configuration of an ODU overhead defined in ITU-TG.709. FIG. 2 is a view illustrating the configuration of a PM overhead defined in ITU-TG.709. FIG. 3 is a view illustrating the configuration of a TCM overhead defined in ITU-TG.709.
As illustrated in FIG. 1, in an OTN, a PM overhead for monitoring a defect to a path between any transmission devices in a network or monitoring the line quality of the path is defined in the tenth to twelfth columns at the third row in an ODU overhead of an OTN frame. In addition, as illustrated in FIGS. 1 and 2, a one bit path delay measurement (DMp) signal indicating the start of delay measurement test for the path is defined in a bit 7 in a PM and TCM field in the third column at the second row. In addition, as illustrated in FIG. 1, TCM overheads (TCM1 to TCM6) for monitoring a defect to a maximum of six TCM sections randomly set on a path or the line quality of the path are defined in the fifth to thirteenth columns at the second row and first to ninth columns at the third row in the ODU overhead. In addition, as illustrated in FIGS. 1 and 2, six one-bit TCM delay measurement (DMti) signals indicating the start of delay measurement test for the six TCM sections are defined in bits 1 to 6 in the PM and TCM field.
The measurement of path delay time in the OTN is performed when a change in the above-described one-bit DM signals (DMp signal and DMti signal) is detected. Specifically, a transmission device configured to measure the path delay time inverts a DM signal in an OTN frame from a current value at the start of measurement. Then, the transmission device transmits the OTN frame including the DM signal having the inverted value to a counterpart device (e.g., a counterpart transmission device) on a transmission path to be measured. In this manner, the value of the DM signal is inverted from 0 to 1 or from 1 to 0 at the start of measurement. The inverted value of the DM signal is held until the start of the next delay measurement test. The OTN frame including the DM signal having the inverted value is received by the counterpart device via the transmission path to be measured and is returned from the counterpart device to the transmission device via the transmission path to be measured. When the DM signal having the inverted value is detected from the frame returned from the counterpart device, the transmission device terminates the measurement of path delay time. Thus, time from a point of time when the value of the DM signal is inverted to a point of time when the DM signal having the inverted value is detected is measured as the path delay time for the transmission path to be measured.
In addition, the following hitless switching system has been known. Specifically, in a device arranged opposite to a termination device (counterpart station), pseudo error information is inserted in a B2 byte of a SDH frame signal in a B2 error insertion unit, and is transmitted to the counterpart station via working and protection transmission paths. In the counterpart station, the number of errors is calculated from the B2 byte information, inserted in a M1 byte, and returned to the device via the working and protection transmission paths. In the device, a path length difference between the working and protection transmission paths is detected using a detection phase difference of the number of M1 byte errors and setting control of delay amount of working and protection transceivers is performed by a phase controller.
In addition, the following SDN transmission node-to-node distance measuring system has been known. Specifically, a measuring unit and a loop-back unit are included in the SDN transmission node-to-node distance measuring system to measure a node-to-node distance. The measuring unit starts measurement of frame round-trip time when a frame having start data inserted in a specific byte of a frame overhead is transmitted. The loop-back unit starts measurement of delay time when the start data are detected from the specific byte of the frame overhead received from the measuring unit, and stops the measurement of delay time when a specific byte of the frame overhead transmitted to the measuring unit is detected. The loop-back unit inserts the measured delay time data in the detected specific byte and returns the specific byte to the measuring unit. The measuring unit stops the measurement of round-trip time when the delay time is separated from the specific byte of the frame overhead received from the loop-back unit, and corrects the round-trip time by subtracting the separated delay time from the measured round-trip time.
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2006-13642, and Japanese Laid-Open Patent Publication No. 2000-332715.